The invention generally relates to sensor systems for detecting electrical signals within subjects, and relates in particular to electrocardiogram detection systems.
Conventional electrocardiogram (ECG) systems generally include an electrically conductive material that provides a conductive path between a surface of a subject and medical instrumentation. Sensors for use in biomedical applications such as ECG applications, are disclosed for example, in U.S. Pat. No. 4,848,353, which discloses an electrically-conductive, pressure sensitive adhesive; U.S. Pat. No. 5,800,685, which discloses an electrically conductive adhesive hydrogel; and U.S. Pat. No. 6,121,508, which discloses a conductive hydrophilic pressure sensitive adhesive.
FIG. 1, for example, diagrammatically shows a conductive sensor device 10 of the prior art that includes an ionically conductive adhesive 12, a conductive electrode 14, and a supporting substrate 16. The ionically conductive adhesive 12 is applied to a patient, and electrical signals within the patient underlying the adhesive 12 travel through the adhesive 12 to the conductive electrode 14, which is electrically coupled to monitoring equipment. Certain ECG systems, for example, employ an ionically conductive hydrogel that includes water soluble salts dispersed therein, and in certain systems, these hydrogels are formulated to also function as the skin attachment adhesive.
Such hydrogels typically contain some amount of water within a gel and require that the material be maintained in a sealed environment (e.g., in sealed packages) until being used. Such materials are generally not re-usable in environments where the humidity is not closely controlled. These limitations adversely affect both the cost of sensors that use such conductive adhesives as well as the amount of use that any particular sensor may enjoy.
The hydrogels perform as signal receptors via an ionically conductive mechanism and are therefore low impedance receptors. For example, the conductive electrode may include silver and silver chloride (Ag/AgCl), which typically has a sheet resistance of between 0.1 and 0.5 Ohms/sq-mil. The units Ohms/sq/mil are conventionally used to refer to surface resistivity (Ohms/square) over a volume, yielding Ohms/sq-mil. The conductive layer is deposited over a conductive carbon coated polymeric film (typically having an impedance range of between 1-1000 Ohms/sq/mil) and a conductive lead that is used to couple the electrode to monitoring equipment. The electrode layer serves as a transducer between the ionically generated biological signal and the electrical signal transmitted in the conducting solution. The chloride serves as the ion in the electrolyte. Current flows freely across the electrode because the Ag/AgCl chemical structures are stable.
When the hydrogel of an electrode is placed in contact with the skin, ions will diffuse into and out of the metal via the hydrogel. Copper has an electrode potential of 340 mV, which is a greater potential than exists in an ECG signal (˜1 mV). The reference electrode should therefore, cancel this potential, but in practice this is not the case. Electrode potentials change with time due to the ionic interaction. Also, any two electrodes and the underlying skin surfaces are not identical. For these reasons the electrode potentials differ. The electrode potentials appear as signal offset. Silver chloride (AgCl) has a potential of under 5 mV, which is easily handled by typical monitoring technology and will not interfere with the ECG signal. For this reason the AgCl produces low levels of noise (less than 10 μV) which is ideal for the ECG application since the amplitude of the heart palpitations that are required to be transmitted to the monitoring equipment.
The number of signal detecting devices used in a harness system may typically range from 3 to 13 electrodes or more. Employing a larger number of detection points provides that many points of reference are available for monitoring a subject, such as a patient's heart. As shown in FIG. 2, some ECG harness systems provide ten or more receptors (electrical contacts) 20 that are coupled to a common harness 22 that leads to an ECG device (not shown) via a connector 24. Harness systems such as shown in FIG. 2 may be easier to hook-up to the ECG monitor than separately-wired sensors, and may be more comfortable for the patient as well as more securely attachable to the patient. Because the hydrogels are low impedance therefore, the ECG harness systems must also be low in electrical impedance.
U.S. Patent Application Publication No. 2004/0000663 discloses a water insensitive alternating current responsive composite that may be used as an adhesive or a polymeric film in a sensor, and provides that an alternating current signal on one side of the composite may be capacitively coupled to the other side of the composite by having the dielectric properties of the material change with the application of an alternating current field (e.g., exhibits dielectric dispersion) such that a charge is released from the composite at the other side of the composite responsive to the changing dielectric properties. The signal receptive materials of U.S. Patent Application Publication No. 2004/0000663 are disclosed to have impedance values of about 100 kΩ or higher.
There remains a need, however, for inexpensive yet effective biomedical sensor harness and wiring systems that may be easily and economically employed in a variety of applications, and that provide improved sensitivity and useful information to a wide variety of medical personnel.